In the presence of a GNOME panel on the top of the screen, Metacity refuses
to allow windows to be dragged up past that panel's bottom edge. This is
arguably
good behavior, except in the case I'm experiencing difficulty with.

In my Xinerama setup, the left head has a top panel, and the right head
does not. Metacity does not let me drag windows up above the bottom of the
extent of the panel - even on the right head, which has no panel. The
result is a strip of pixels on the right head that cannot be occupied by
any normal windows and is only useful for placing a panel (which I'd prefer
not to have.)

Is it possible to make this movement restriction specific to the head(s) on
which a top panel exists?

I have experienced this problem under GNOME 2 with both the special 'menu'
panel and with a normal edge panel; I have not tried to reproduce under
GNOME 1.